Fuel Cell Stack with Improved End Cell Performance

ABSTRACT

A fuel cell stack that includes a gas diffusion media for the end cells in the stack that has less of an intrusion into the flow field channels of the end cells that the other cells, so as to increase the flow rate through the flow channels in the end cells relative to the flow rate through the flow channels in the other cells. A different diffusion media can be used in the end cells than the nominal cells, where the end cell diffusion media has less of a channel intrusion as a result of diffusion media characteristics. Also, the same diffusion media could be used in the end cells as the nominal cells, but the end cell diffusion media layers could be thinner than the nominal cell diffusion media layers. Further, a higher amount of pre-compression can be used for the diffusion media in the end cells.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a fuel cell stack having improvedend cell performance and, more particularly, to a fuel cell stack havingimproved end cell performance by providing a diffusion media for the endcells that has less of an intrusion into the flow field channels thanthe diffusion media for the flow channels of the other cells so as toincrease the flow rate through the flow channels in the end cellsrelative to the flow rate through the flow channels in the other cells.

2. Discussion of the Related Art

Hydrogen is a very attractive fuel because it is clean and can be usedto efficiently produce electricity in a fuel cell. A hydrogen fuel cellis an electro-chemical device that includes an anode and a cathode withan electrolyte therebetween. The anode receives hydrogen gas and thecathode receives oxygen or air. The hydrogen gas is dissociated in theanode to generate free protons and electrons. The protons pass throughthe electrolyte to the cathode. The protons react with the oxygen andthe electrons in the cathode to generate water. The electrons from theanode cannot pass through the electrolyte, and thus are directed througha load to perform work before being sent to the cathode.

Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell forvehicles. The PEMFC generally includes a solid polymer electrolyteproton conducting membrane, such as a perfluorosulfonic acid membrane.The anode and cathode typically include finely divided catalyticparticles, usually platinum (Pt), supported on carbon particles andmixed with an ionomer. The catalytic mixture is deposited on opposingsides of the membrane. The combination of the anode catalytic mixture,the cathode catalytic mixture and the membrane define a membraneelectrode assembly (MEA).

Several fuel cells are typically combined in a fuel cell stack togenerate the desired power. The fuel cell stack receives a cathodereactant gas, typically a flow of air forced through the stack by acompressor. Not all of the oxygen is consumed by the stack and some ofthe air is output as a cathode exhaust gas that may include water as astack by-product. The fuel cell stack also receives an anode hydrogenreactant gas that flows into the anode side of the stack. The stack alsoincludes flow channels through which a cooling fluid flows.

The fuel cell stack includes a series of bipolar plates positionedbetween the several MEAs in the stack, where the bipolar plates and theMEAs are positioned between two end plates. The bipolar plates includean anode side and a cathode side for adjacent fuel cells in the stack.Anode gas flow channels are provided on the anode side of the bipolarplates that allow the anode reactant gas to flow to the respective MEA.Cathode gas flow channels are provided on the cathode side of thebipolar plates that allow the cathode reactant gas to flow to therespective MEA. One end plate includes anode gas flow channels, and theother end plate includes cathode gas flow channels. The bipolar platesand end plates are made of a conductive material, such as stainlesssteel or a conductive composite. The end plates conduct the electricitygenerated by the fuel cells out of the stack. The bipolar plates alsoinclude flow channels through which a cooling fluid flows.

The membrane within a fuel cell needs to have a certain relativehumidity so that the ionic resistance across the membrane is low enoughto effectively conduct protons. This humidification may come from thestack water by-product or external humidification. The flow of thereactant gas through the flow channels has a drying effect on themembrane, most noticeably at an inlet of the flow channels. Also, theaccumulation of water droplets within the flow channels from themembrane hydration and water by-product could prevent reactant gas fromflowing therethrough, and cause the cell to fail, thus affecting thestack stability. The accumulation of water in the reactant gas flowchannels is particularly troublesome at low stack output loads.

The end cells in a fuel cell stack typically have a lower performance asa result of cell stability than the other cells in the stack.Particularly, the end cells are more exposed to ambient temperature, andthus have a temperature gradient that causes them to operate at a lowertemperature as a result of convective heat losses. Because the end cellsare typically cooler than the other cells in the stack, water vapor moreeasily condenses into liquid water so that the end cells have a higherrelative humidity, which causes water droplets to more readily form inthe flow channels of the end cells. Also, at low stack loads thetemperature of the cooling fluid is reduced, which reduces thetemperature of the stack and typically increases the relative humidityof the reactant gas flow.

End cell stability can be illustrated by the following example. A fuelcell stack may be operating at a certain current density, such as 0.6A/cm², which provides a cell voltage of about 0.7 volts. The flow rateof reactant gas through the reactant gas flow channels to produce thiscurrent density is sufficient to force accumulated water in the flowchannels out of the flow channels. If the current density of the stackis reduced to 0.1 A/cm², such as for vehicle idle, the cell voltageincreases to about 0.85 volts, and the flow rate of reactant gas throughthe flow channels is significantly reduced. Because the end cells are ata lower temperature, more water will condense in the end cell flowchannels increasing channel blockage. Because the channels may beblocked with water, the reactant gas may be diverted to the flowchannels of other fuel cells, which causes the voltage of the end cellsto become unstable.

It is known in the art to heat the end cells with resistive heaterspositioned between the unipolar plate and the MEA so as to compensatefor convective heat losses. These known systems typically attempted tomaintain the end cell temperature the same as the other cells in thestack by monitoring the temperature of the cooling fluid out of thestack. However, end cell stability is still a problem even with theaddition of such heaters.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a fuel cellstack is disclosed that includes a gas diffusion media for the end cellsin the stack that has less of an intrusion into the flow field channelsof the end cells than the other cells, so as to increase the flow ratethrough the flow channels in the end cells relative to the flow ratethrough the flow channels in the other cells. A different diffusionmedia can be used in the end cells than the nominal cells, where the endcell diffusion media has less of a channel intrusion as a result ofdiffusion media characteristics, such as having a higher modulus ofelasticity, a higher shear modulus, a lower compressibility, etc. Also,the same diffusion media could be used in the end cells as the nominalcells, but the end cell diffusion media layers could be thinner than thenominal cell diffusion media layers, which will provide a reduced flowchannel intrusion. Further, a higher amount of pre-compression can beused for the diffusion media in the end cells.

Additional features of the present invention will become apparent fromthe following description and appended claims, taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a series of end cells in a fuel cellstack;

FIG. 2 is a graph with increase in hydraulic diameter on the horizontalaxis and flow rate increase in end cells on the vertical axis showing anincrease of reactant gas flow in the end cells of a fuel cell stack withvarious increases in hydraulic diameter; and

FIG. 3 is a graph with diffusion media material on the horizontal axisand diffusion media channel intrusion on the vertical axis.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed toa fuel cell stack including a gas diffusion media for the end cells inthe stack that has a reduced intrusion into the flow field channels soas to increase the flow rate through the flow channels in the end cellsrelative to the flow rate through the flow channels in the other cellsis merely exemplary in nature, and is in no way intended to limit theinvention or its applications or uses.

FIG. 1 is a cross-sectional view of a series of end cells 10 in a fuelcell stack of the type discussed above. The number of end cells thatwould benefit from the invention would depend on the certain stackdesign, and would typically be up to five fuel cells. Four of the endcells 10 are shown in FIG. 1. Each fuel cell 10 includes a cathode side12 and an anode side 14 separated by a polymer electrolyte membrane 16.A cathode side diffusion media layer 20 is provided on the cathode side12, and a cathode side catalyst layer 22 is provided between themembrane 16 and the diffusion media layer 20. Likewise, an anode sidediffusion media layer 24 is provided on the anode side 14, and an anodeside catalyst layer 26 is provided between the membrane 16 and thediffusion media layer 24. The catalyst layers 22 and 26 and the membrane16 define an MEA. The diffusion media layers 20 and 24 are porous layersthat provide for input gas transport to and water transport from theMEA. Various techniques are known in the art for depositing the catalystlayers 22 and 26 on the diffusion media layers 20 and 24, respectively,or on the membrane 16.

A cathode side flow field plate or bipolar plate 28 is provided on thecathode side 12 and an anode side flow field plate or bipolar plate 30is provided on the anode side 14. The bipolar plates 28 and 30 areprovided between the fuel cells in the fuel cell stack. A hydrogenreactant gas flow from flow channels 32 in the bipolar plate 30 reactswith the catalyst layer 26 to dissociate the hydrogen ions and theelectrons. Reactant gas flow from flow channels 34 in the bipolar plate28 reacts with the catalyst layer 22. The hydrogen ions are able topropagate through the membrane 16 where they carry the ionic currentthrough the membrane 16. The end product is water, which does not haveany negative impact on the environment.

In this non-limiting embodiment, the bipolar plate 28 includes twostamped metal sheets 36 and 38 that are welded together. The sheet 38defines the flow channels 34 and the sheet 36 defines flow channels 40for the anode side of an adjacent fuel cell to the fuel cell 10. Coolingfluid flow channels 42 are provided between the sheets 36 and 38, asshown. Likewise, the bipolar plate 30 includes a sheet 44 defining theflow channels 32, and a sheet 46 defining flow channels 48 for thecathode side of an adjacent fuel cell. Cooling fluid flow channels 50are provided between the sheets 44 and 46, as shown. The bipolar plates28 and 30 can be made of any suitable conductive material that can bestamped, such as stainless steel, titanium, aluminum, etc.

The present invention proposes a technique for increasing end cellperformance and reducing end cell instability by reducing diffusionmedia intrusion into the flow channels in the end cells for one or bothof the cathode and anode side of the fuel cell to increase the flow ratethrough the flow channels relative to the flow rate through the flowchannels of the other cells. As is know in the art, because thediffusion media material is soft it partially enters the flow channelswhen the stack is assembled and the cells are compressed together,referred to in the industry as diffusion media intrusion. This flowchannel intrusion by the diffusion media reduces the flow through thechannel.

The present invention contemplates various techniques for reducing thediffusion media intrusion into the flow channels in the end cells so asto increase the flow rate of the reactant gas through the flow channelsin the end cells relative to the flow rate of the reactant gas throughthe flow channels in the other or nominal cells. A different diffusionmedia can be used in the end cells than the nominal cells, where the endcell diffusion media has less of a channel intrusion as a result ofdiffusion media characteristics, such as having a higher modulus ofelasticity, a higher shear modulus, a lower compressibility, etc. Themodulus of elasticity can be determined by a three point bend test, theshear modulus can be determined by a transverse shear test and acompression test can be performed in the thickness direction. Also, thesame diffusion media could be used in the end cells as the nominalcells, but the end cell diffusion media layers could be thinner than thenominal cell diffusion media layers, which will provide a reduced flowchannel intrusion. Further, a higher amount of pre-compression can beused for the diffusion media in the end cells. It has previously beenproposed in the art to compress the diffusion media before it isassembled in the stack to provide a better diffusion media layerthickness uniformity. The pre-compression refers to a procedure tocompress the gas diffusion media in the thickness direction to achievehigher density. The method of pre-compression may include, but not belimited to, applying a uniform static or dynamic compressive load overthe plane of the gas diffusion media and calendaring the gas diffusionmedia through the nips of calendar rollers.

A model based on the Hagan-Poiseuille equation for incompressiblelaminar flow and cylindrical conduit can be used to estimate the effectof the gas diffusion media intrusion into the flow field channels onflow distribution. This equation is given as:

$\begin{matrix}{{- \frac{p}{x}} = {32\frac{\mu \; U_{avg}}{D^{2}}}} & (1)\end{matrix}$

where P is the reactant pressure, μ is the reactant viscosity, D is thehydraulic diameter of the flow channel, and U_(avg) is the averagevelocity of the gas flowing through the flow channel.

For sake of simplicity, it is assumed that a single channel representseach end cell. In the case of an uniform flow and pressure distributionin both the inlet and outlet manifolds, the fuel cell stack can berepresented by a set of n parallel cells of the same length with somenominal hydraulic diameter D and nominal flow rate per channel Q.Therefore, all of the fuel cells will have the same pressure drop as:

$\begin{matrix}{\Delta \; {\left. P \right.\sim\frac{Q}{D^{4}}}} & (2)\end{matrix}$

As a result of the reduced gas diffusion media intrusion into the flowfield channels, the hydraulic diameter D in each of the m end cellsincreases by ΔD compared to the nominal cells. In this case, the flowthrough each of the end cells will increase by an amount of ΔQ. Becausethe system is maintaining a constant flow rate, the total amount of flowrate increase in the end cells (mΔQ) will be provided by the remainingn−m cells. Provided that the pressure drop in the nominal cells and theend cells is still equal, and the change in channel hydraulic diameter Dof the end cells is relatively small, the following equation can beprovided:

$\begin{matrix}{\frac{Q - {m\; \Delta \; {Q/\left( {n - m} \right)}}}{D^{4}} = \frac{Q + {\Delta \; Q}}{\left( {D + {\Delta \; D}} \right)^{4}}} & (3)\end{matrix}$

Dividing equation (3) by

$\frac{Q}{D^{4}},$

and substituting

$\begin{matrix}{{\delta = {{\frac{\Delta \; Q}{Q}\mspace{14mu} {and}\mspace{14mu} ɛ} = \frac{\Delta \; D}{D}}},} & {{equation}\mspace{14mu} (3)}\end{matrix}$

can be rewritten in a dimensionless form as:

$\begin{matrix}{{1 - \frac{m\; \delta}{\left( {n - m} \right)}} = \frac{\left( {1 + \delta} \right)}{\left( {1 + ɛ} \right)^{4}}} & (4)\end{matrix}$

Solving equation (4) for δ, the increased flow in the end cell can beobtained as a percentage to the nominally expected flow as:

$\begin{matrix}{{\delta \left( {n,ɛ} \right)} = \frac{\left( {n - m} \right)\left\lbrack {\left( {1 + ɛ} \right)^{4} - 1} \right\rbrack}{\left( {n - m} \right) + {m\left( {1 + ɛ} \right)}^{4}}} & (5)\end{matrix}$

Equation (5) is represented in FIG. 2 for a fuel cell stack having 200cells. The increased flow in the end cells can be easily provided by theremaining 200−m cells, which results in a significant increase in theend cell flow rates. It has been discovered from this graph that aslight increase in hydraulic diameter D of 5% in the end cell can easilycreate a relatively large increase of 21% in the flow rate. Further, anincrease in the hydraulic diameter D of 10% can provide a 46% increasein end-cell flow rate.

Using equation (5) and FIG. 2 for a flow field with a channel depth of0.25 mm and a channel width of 1.0 mm using a SGL25BC gas diffusionmedia with a nominal gas diffusion media intrusion of 0.05 mm, it hasbeen discovered that to achieve a flow rate increase of 20% would onlyrequire using a gas diffusion media having channel intrusions of 0.04mm.

FIG. 3 illustrates the gas diffusion media channel intrusion for variousgas diffusion medias. Using this graph, a particular diffusion media canbe selected to meet the desired flow characteristics based on thediscussion above. In FIG. 3, the bars from left to right are representedby the following diffusion media materials in order.

Toray TGP-H-060 Toray TGP-H-090 SGL GDL 20BC GL GDL 21BC SGL GDL 25BCMitsibushi Rayon MRC 105 Ballard AvCarb P50

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

1. A fuel cell stack comprising: a plurality of fuel cells including atleast one end cell at ends of the fuel cell stack; a plurality of flowfield plates separating the fuel cells in the fuel cell stack, said flowfield plates including flow channels; and a plurality of diffusion medialayers positioned adjacent to the flow field plates in the fuel cells,wherein the diffusion media material of the diffusion media layers isselected so that the diffusion media material has a lower intrusion intothe flow channels of the flow field plates in the end cells of the fuelcell stack than the intrusion into the flow channels of the flow fieldplates in the other cells.
 2. The fuel cell stack according to claim 1wherein the at least one end cell is five or less end cells at each endof the stack.
 3. The fuel cell stack according to claim 1 wherein thediffusion media material of the diffusion media layers in the end cellsis different than the diffusion media material of the diffusion medialayers in the other cells.
 4. The fuel cell stack according to claim 1wherein the diffusion media material of the diffusion media layers inthe end cells is the same as the diffusion media material of thediffusion media layers in the other cells.
 5. The fuel cell stackaccording to claim 1 wherein the diffusion media layers in the end cellsare thinner than the diffusion media layers in the other cells.
 6. Thefuel cell stack according to claim 1 wherein the diffusion mediamaterial of the diffusion media layers in the end cells has a highermodulus of elasticity than the diffusion media material of the diffusionmedia layers in the other cells.
 7. The fuel cell stack according toclaim 1 wherein the diffusion media material of the diffusion medialayers in the end cells has a greater shear modulus than the diffusionmedia material of the diffusion media layers in the other cells.
 8. Thefuel cell stack according to claim 1 wherein the diffusion mediamaterial of the diffusion media layers in the end cells has a lowercompressibility than the diffusion media material of the diffusion medialayers in the other cells.
 9. The fuel cell stack according to claim 1wherein the diffusion media material of the diffusion media layers inthe end cells has been exposed to a greater pre-compression than thediffusion media material of the diffusion media layers in the othercells.
 10. A fuel cell stack comprising a plurality of diffusion medialayers positioned adjacent to flow field plates between fuel cells inthe stack, wherein the diffusion media material of the diffusion medialayers is selected so that the diffusion media material has a lowerintrusion into the flow channels in the flow field plates in end cellsof the fuel cell stack than the intrusion of the diffusion media intothe flow channels of the flow field plates of other cells.
 11. The fuelcell stack according to claim 10 wherein the number of end cells is fiveor less end cells at each end of the stack.
 12. The fuel cell stackaccording to claim 10 wherein the diffusion media material of thediffusion media layers in the end cells is different than the diffusionmedia material of the diffusion media layers in the other cells.
 13. Thefuel cell stack according to claim 10 wherein the diffusion mediamaterial of the diffusion media layers in the end cells is the same asthe diffusion media material of the diffusion media layers in the othercells.
 14. A method for increasing the stability of end cells in a fuelcell stack comprising selecting a diffusion media for fuel cells in thefuel cell stack so that the intrusion of the diffusion media into flowchannels in end cells is less than the intrusion of the diffusion mediainto flow channels of other fuel cells.
 15. The method according toclaim 14 wherein the diffusion media in the end cells is different thanthe diffusion media in the other cells.
 16. The method according toclaim 14 wherein the diffusion media material in the end cells isdifferent than the diffusion media material in the other cells.
 17. Themethod according to claim 14 wherein the diffusion media in the endcells is thinner than the diffusion media in the other cells.
 18. Themethod according to claim 14 wherein the diffusion media in the endcells has a higher modulus of elasticity than the diffusion media in theother cells.
 19. The method according to claim 14 wherein the diffusionmedia in the end cells has a greater shear modulus than the diffusionmedia in the other cells.
 20. The method according to claim 14 whereinthe diffusion media in the end cells has a lower compressibility thanthe diffusion media in the other cells.
 21. The method according toclaim 14 wherein the diffusion media in the end cells has been exposedto a greater pre-compression than the diffusion media in the othercells.